1. Field of the Invention
This invention relates to a method of producing a discharge display device and more particularly to a method of forming a LaB.sub.6 cathode for the discharge display device.
2. Description of the Prior Art
Recently, development of discharge display devices, especially direct current type XY matrix discharge display panels termed plasma display panels, or PDP, has been promoted. Fundamental problems in this development are both of improvement in discharge efficiency, that is, achievement of high luminance with low power consumption, and increasing life of the discharge display panel by stabilizing electrodes and the other materials as regards their physical and chemical properties. Research in the area of electrode (especially, cathode) materials and structures is important to the solution of the problems.
Nickel (Ni) is conventionally used as an anode and a cathode. Ni has little resistance against discharge sputtering, and therefore a Ni cathode deteriorates in several seconds of operation. To cope with this, in prior art arrangements, mercury (Hg) has been sealed in the discharge display panel and deposited on a surface of the electrode to suppress sputtering. However, when mercury (Hg) is sealed in the discharge display panel, it is difficult to maintain discharge characteristics of each display cell uniform over a long time in the discharge display panel with a large capacity, as non-distribution of the mercury occurs due to change on standing.
Further, when such a discharge display panel is used in a closed room such as a cockpit, mercury cannot be used due to health hazards.
Meanwhile, lanthanum boride (LaB.sub.6) had been proposed as a cathode material. LaB.sub.6 has advantages that its work function is low (.gamma. coefficient is large) and discharge efficiency is high; and it is superior in physical and chemical stability due to its covalent bonding structure.
However, a LaB.sub.6 cathode has not yet reached practical use for the reason that its usual production process employing a thin-film evaporation method or a plasma spraying method, is complicated and results in increase in cost. Particularly, it is difficult to form a relatively uniform electrode with a large capacity and a large screen. Another reason is that the electrode cannot be formed in connection with the other panel structure by a thick-film printing method with a low cost.
In the case where a LaB.sub.6 cathode is intended to be formed by the thick-film printing method, it is generally burnt in the atmosphere of nitrogen N.sub.2 at 800.degree.-900.degree. C. after printing and application. However, as the substrate of the discharge display panel is glass, the temperature is permitted to be raised up to about 600.degree. C., and as the structure such as the other electrodes and barrier is oxide, such a burning step is usually carried out in air. For these reasons, it is difficult to form the LaB.sub.6 cathode. In addition, LaB.sub.6 has a high melting point of about 2300.degree. C., and therefore it cannot be sintered at a temperature of about 600.degree. C. with the result that resistance after formation of the cathode is disadvantageously increased to 10.sup.9 .OMEGA. and more. In case that the thick-film printing method is adopted, a binder substance such as frit glass is generally mixed with LaB.sub.6 powder so as to obtain a bonding strength between each of the LaB.sub.6 powder particles. However, it is considered impractical to use glass binder mixed with LaB.sub.6 powder since it causes high resistance after formation of the LaB.sub.6 cathode.
On the other hand, the present inventors have developed a method of forming a LaB.sub.6 cathode which enables the LaB.sub.6 cathode to be formed by a thick-film printing method. See copending related application Ser. No. 721,955, filed concurrently. According to that method, a LaB.sub.6 paste is prepared by using an ionic conductive alkali glass as a glass binder, and the LaB.sub.6 paste is applied and printed onto a base electrode such as Ni, thereafter burning the same in the air at 500.degree.-600.degree. C. Then, after such steps as frit sealing, heating exhaustion, gas sealing and final sealing of the discharge display panel, voltage is applied between an anode and a cathode to effect activation treatment by gas discharge with large current. With this activation treatment, no glass becomes present on the LaB.sub.6 layer, and LaB.sub.6 is exposed to the surface of the LaB.sub.6 layer. Simultaneously, a surface of each LaB.sub.6 particle is fused and bound with other particles, thus forming the LaB.sub.6 cathode.
However, it is preferred that glass binder not be contained in the LaB.sub.6 paste. This is due to the fact that as the surface of the LaB.sub.6 particles and the space therebetween is covered or filled with glass binder, it is difficult to form an electrical conductive path, resulting in difficulty in activation of the electrodes, and that in case of using a frit glass containing lead (Pb) as the binder, there is a possibility that the life endurance characteristic will be reduced by sputtering of metallic Pb as desposited.